This invention relates in general to fuser assemblies for reproduction apparatus, and more particularly to detection of erratic resistance behavior in temperature sensors for a reproduction apparatus fuser assembly.
In typical commercial electrographic reproduction apparatus (copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photoconductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the dielectric support member. A receiver member, such as a sheet of paper, transparency or other medium, is then brought into contact with the dielectric support member, and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by heat and pressure to form a permanent reproduction thereon.
One type of fuser assembly, utilized in typical reproduction apparatus, includes at least one heated roller and at least one pressure roller in nip relation with the heated roller. The fuser assembly rollers are rotated to transport a receiver member, bearing a marking particle image, through the nip between the rollers. The pigmented marking particles of the transferred image on the surface of the receiver member soften and become tacky in the heat. Under the pressure, the softened tacky marking particles attach to each other and are partially imbibed into the interstices of the fibers at the surface of the receiver member. Accordingly, upon cooling, the marking particle image is permanently fixed to the receiver member.
Fusing the marking particle image onto a receiver member generally requires a precisely controlled temperature of the surface of the fusing roller. Typical temperatures of 300xc2x0 F. to 430xc2x0 F. are used depending on the design of the system and must be held to within +/xe2x88x9210xc2x0 F. of the set point for optimum results from the particular fuser assembly. If the fuser temperature varies high or low from the set point, many undesirable effects are possible. If the fuser temperature is too low, the marking particles will not be melted sufficiently for proper fixing to the receiver member. This causes poor quality images where portions of the image flake off the receiver during normal handling. In addition, marking particles may adhere to the fuser roller and later flake off randomly on successive receiver members causing black/colored artifacts (spots) on the receiver member. When the buildup of marking particles on the fuser roller becomes too great, the receiver member may stick to the fuser roller and cause a jam. In most cases, this series of events cause the fuser roller and other parts of the fuser assembly to be irreparably damaged to the extent that they must be replaced.
If the fuser temperature is too high, the image will be fixed to the receiver but marking particles may again adhere to the fuser roller, causing poor image quality and jams. Other high fuser temperature effects include, but are not limited to, high internal temperatures inside the enclosure of the machine which may damage temperature sensitive components such as the dielectric support member, high temperatures of customers accessible parts, general shortening of reproduction apparatus part lives, and evoking the function of the back-up temperature safety systems.
Thermistors are typically used as temperature measuring devices for fuser rollers. These thermistors are enclosed in a holder that is positioned near or in contact with the fuser roller. The assembled thermistor and holder are referred to as the temperature sensor. The thermistor is used in an electrical circuit where its resistance causes a proportional voltage output. As the fuser temperature changes, the thermistor resistance and associated circuit voltages change accordingly. Because of the close proximity of the sensor to the heated fuser roller, the sensor is heated to a high temperature. It has, however, been discovered that some sensors fail in an erratic and intermittent manner. The intermittent nature of the failure makes it difficult for service personnel to identify the failure.
In view of the above, this invention is directed to a method for detecting erratic resistance in the temperature sensor in a reproduction apparatus having a logic and control unit, and a fuser assembly including a heated fuser roller and a sensor for sensing the temperature of such fuser roller by utilizing heat to change sensor resistance to provide an electrical signal corresponding to the fuser temperature, for the logic and control unit. The method includes the steps of, at preselected time intervals, comparing a current temperature reading to a previous temperature reading, and determining if the temperature sensing system indicates that the temperature has changed by more than a predetermined amount based upon reproduction apparatus operating characteristics. If the temperature sensing system indicates, from the determining step, that the temperature has changed by more than a predetermined amount, an indication that an error has occurred is made. Then, such error indication is recorded in an error log of the logic and control unit, and a response strategy for such error is provided.